what are we talking about stars yeah
you always talk about stars i know yeah
so this was just a question that came
out and we're just chatting about this
and we're talking about the kinds of
stars there are and in particular you
know big stars little stars all right
what's the biggest star that's all i
care about is that what you want okay we
can do the biggest star there it is
doesn't look terribly inspiring that's
because it's quite a long way away it
has the inspiring name of r136a1
it's actually in the large magellanic
cloud which is one of our nearby
neighbours so the biggest star that we
know is not even in the milky way
absolutely not there's two reasons why
it's hard to find a really big star in
the milky way one is when you're looking
in the milky way you're always peering
through all the merc all the dust that's
there between us and the center of the
galaxy whereas the large magnetic cloud
is conveniently well out of the plane in
the milky way so we can actually see it
the other thing is that there's a lot of
very strong star formation going on
in the large magellanic cloud very
massive stars are very rare and so
actually you need to have an awful lot
of star formation happening for there to
be just one of these popping up and
secondly they don't live very long they
live a few million years which means
that actually you've really got to at
least in astronomical terms catch them
just at the moment they're forming
because actually if you blink and you
miss it in an astronomical term so
actually you've really got to see the
active star formation that's happening
right now so r136 is the general region
it's in a is the star cluster within
that region and one because it's the
brightest star in that star cluster it's
about 300 times the mass of the sun
which is big and it's about i think it's
about 10 million times as bright as the
sun when i were allowed it was thought
that there weren't any stars more
massive than about 100 150 solar masses
and there is actually there's a physical
reason why you might expect there to be
a limit for how big a star can be which
is that so what makes a star is gravity
it pulls it together but there is a kind
of a
countervailing force an outward force
which is just due to the starlight
itself the radiation pressure from the
star itself tries to push the star apart
so the light itself is trying to push
the star apart
and the interesting thing is as you go
up to more and more massive stars
the mass goes up they get more massive
but they get a lot more luminous so if
you add a bit more mass to a star it
gets a bit more massive but quite a lot
more luminous and that means that even
though in a star like the sun this
radiation pressure really doesn't matter
very much gravity always wins over the
radiation pressure as you go up in mass
the mass goes up so the gravity goes up
but the brightness the luminosity goes
up a lot more and so actually the
radiation pressure becomes more and more
important
and a fairly naive calculation says that
the crossover point where radiation
pressure winds
over gravity so the star would literally
blow itself apart by its own radiation
by its own starlight is about 150 solar
masses i sent paul crowther the guy who
did this an email saying why is it that
there are now stars more massive than we
thought this limit ought to be and he
says the explanation is actually quite
interesting that although for lower mass
stars as you go up a bit in mass the
luminosity goes up a lot when you get to
these very very high mass stars it turns
out the two go up together that the mass
goes up a bit and the luminosity goes up
a bit
so there's no longer this kind of
crossover point where the the radiation
pressure wins in the end so actually
these very massive stars
you haven't quite reached that crossover
point and then you get never get any
closer to it because gravity and the
luminosity go up together in the same
way as it evolves it tends to lose mass
stuff gets thrown off the outside so it
becomes less massive and it gets
brighter so actually these very massive
stars even though they can form
eventually they will cross over that
line because suddenly they get brighter
and less massive so suddenly the
radiation pressure wins out over the
gravity and they will blow themselves
apart quite quickly let me ask you the
question how do you know something's a
star right what's a star
well a star is like a bowl of
mainly hydrogen gas
that has like
spontaneously started
nuclear fusion so you're requiring
that's the interesting definition part
of the definition is to whether you have
nuclear fusion in there of course by
that definition a white dwarf isn't a
star because it doesn't have nuclear
fusion anymore it did at some point in
its life but not any longer there's no
longer a source of energy in it it's
just glowing because it's hot but for
example jupiter gives out more light
than it absorbs it actually glows in the
dark and not because of nuclear fusion
but just because actually it's very
slowly contracting with time centimeter
or so per year and that gravitational
contraction is actually liberating
energy liberating the potential energy
so actually if jupiter emits more light
than it absorbs it turns out both these
ends of the spectrum are hard to
actually detect the high mass stars are
hard to detect because they're very rare
and they don't live very long the low
mass stars they're plentiful there's
loads and loads of them around but
they're so faint that they're actually
hard to spot and actually it's only in
our neighborhood that we can find them
because they're so pathetically faint so
here is the lowest mass star known there
it is wow it's called scr
1845-6357 that's about 15 light years
away cool it turns out actually it has a
companion it's a brown dwarf and we see
this one because we've seen the
reflected light from the star but it's
very very faint we're seeing a brown
dwarf that we normally wouldn't see
because it's being shone upon by its
neighbor the sun the most important star
yeah where is that in the distribution
of star sizes boringly in the middle it
really is it really is well not quite in
the middle so if you think about it so
the lowest mass so usually you think
about these things on kind of a
logarithmic scale the lowest mass stars
are about a tenth the mass of the sun
and the highest mass stars are a few
hundred times the mass of the sun so
it's not quite in the middle it's sort
of somewhat towards the lower mass end
which i guess you sort of expect because
there's more stars towards that end what
about this pistol star pistol star is
about 150 times the mass of the sun it's
one of those that's the kind of the
brightest ones in the milky way it might
well be the biggest star in the milky
way the most massive star in the milky
way and it's quite spectacular to look
at too the interesting thing about these
very massive stars is that they tend to
be in regions of intense star formation
so there's lots of gas around them so
they light it up and look very nice and
actually the other thing you sometimes
find with them is because they tend to
blow off their outer layers because
they're not very well bound together you
sometimes find features around them that
have to do with that as well one of the
things that people are interested in is
the first stars that formed and what
properties they had although we've never
observed them yet the prejudices the
expectation is that the first stars to
form in the universe were very massive
stars they have lifetimes of a few
million years so they wouldn't have hung
around very long but they would have
seeded the universe with heavy elements
so you start off with a big bang which
makes your hydrogen and helium a bit of
lithium but anything heavier than that
you need stars for the oldest stars that
we can see around us today actually have
heavy elements in them so we know they
weren't the first generation of stars
they're the the earliest the kind of
second generation of stars there's two
at least two reasons to think that those
first generations of stars might have
been very massive one is to make a star
you need gas clouds to kind of fragment
and collapse and to get something to
fragment and collapse you need to be
able to cool down and one of the things
that the heavy elements do is they sort
of accelerate the cooling process they
allow gas to cool down and so with heavy
elements you can kind of create quite
low temperature regions and actually
have little things collapsing to make
little stars if you don't have those
heavy elements
it's very hard for gas to cool down
which means you tend to make big things
because it just doesn't fragment into
those little bits
so we think probably that just that
process of collapse would probably tend
to make bigger stars in the early
universe because it's hard for little
things to cool down and collapse because
you don't have the heavy elements the
other is that process we were talking
about about that competition between
gravity and radiation but what's
happening is the radiation is is kind of
driving its way out and smacking into
things
and
the the interactions are much stronger
if you have the heavy elements there
the opacity the the how opaque the star
is depends enormously on how much of
those heavy elements there are so stars
are much more transparent if you don't
have heavy elements which means that
rather than pushing the star apart that
radiation tends to be able to escape
more easily so that process competition
between the gravity pulling things in
and the radiation pushing it apart if
you don't have the heavy elements it's
harder for the radiation to push things
apart
so therefore you can actually make
bigger and bigger stars so that side of
the process
tends to
make more massive stars stable
if you haven't got the heavy elements as
we did in the early parts of the
universe we won't be able to see them
will we by looking back in time that's
the plan yeah so next generation of
telescopes james webb space telescope
next generation of ground-based
telescopes next generation of space
telescopes we really want to be looking
for those first stars to form and we
will actually have the sensitivity to
find them particularly if they're very
massive because of course very massive
things are very bright so hopefully we
will in you know on the timescale of
maybe a decade or so be able to see
those first stars as they formed wow
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